MINERALS AND GEOLOGY OF CANADA. 27 
called the gladella. It is bounded laterally bythe two longitudinal 
farrows mentioned above; and is either smooth, or variously lobed, 
furrowed, or granulated. In some genera it expands anteriorly, or 
towards the upper part; and in others it becomes contracted in this 
direction. The head-shield in most genera exhibits also on each side 
of the glabella a sutural line—called, technically, the facial suture— 
as shewn at f fin Fig. 138. The direction of the facial suture differs 
somewhat in different genera, as explained in our descriptions of these, 
below. In some few (as in Trinuclens) again, it is either absent, or con- 
cealed by being situated along the edge of the shield. The eyes (e e) 
when present, occur on each side of the head-shield, in the line of the 
facial suture, as shewn in the figure. They are compound, as in ex- 
isting crustaceans, insects, &c. ; and the component facets in certain 
genera (Dalmannites, Phacops) are thrown up in strong relief, form- 
ing the so-called reticulated eye. In other trilobites the reticulation 
is less distinct.* The sides of the head-shield or “cheeks,” (c c), 
often separate along the facial suture, and are found detached. The 
shell is continued over the head-shield ; and under the glabella, where 
the mouth was situated, a so-called hypostoma or labrum is occasionally 
found. ‘This, which is also and more commonly met with in a de- 
tached state, is generally of an oval form; but in the genus Asaphus 
(see below) it is hollowed out into a fork, or is somewhat of a horse- 
shoe shape. The hinder or lower extremities of the head-shield are 
rounded in some species, whilst in others they terminate in long or 
short horns. 
The body or thorax of the trilobite is composed of a series of sepa- 
rate rings or segments, varying in number in different genera. Each 
segment is sub-divided into three parts by the two longitudinal fur- 
rows already alluded to. The middle part, or that between the 
furrows, is generally known as the axis, whilst the outside portions 
are called sides or pleure. These latter have their ends rounded in 
some species, and pointed, or even prolonged into spines, in others. 
Ta some, also, there is a raised band on each pleura, and, in others, a 
groove or furrow. Detached segments, or the three-curved impres- 
sions of these, shewing their trilobed character, are frequently seen in 
our Utica Slate and other fossiliferous rocks. The greater or less 
degree of mobility with which the thoracic segments were endowed, 
simple stomata in merely approximate union. See an article by the writer, on the classifi- 
cation-characters, &¢c., of the Trilobites, in this Journal, Vol. 1., pp. 271-286. 
